1. Field of the Invention
This invention relates generally to isotropic electrically conductive bonding between two metal surfaces, and more particularly, to a method and structure of forming a palladium matrix in situ within adhesive material to provide mechanical bonding and electrical connection between two metal surfaces.
2. Description of the Prior Art
Conductive adhesives are often used to mechanically bond two articles together while providing electrical conductivity between metal surfaces on the faces of the articles. These conductive adhesives may be selected from various types of thermoplastic: material, one of the materials being a polyimide/siloxane. In order to achieve the high conductivity often required, a very high loading of the metal particles within the adhesive: is required. Typically, these particles are silver particles or, in some cases, tin coated copper may be used. Moreover, after the bond is formed, underfill material is provided around the joints which tends to degrade the electrical interconnection of the particles to the metal surface to some extent. In addition, contact resistance: often in the form of an oxide film on the: metal surface can commonly occur at the interface where the conductive adhesive comes into contact with the metal surface of the item being joined. Also, the polymer or the oxidation of the metal surface itself may cause an insulative layer to be formed at the interface between the metal particles and the metal surface immediately or after latency contact with the metal surface of the item being joined. Also, the polymer or the oxidation of the metal surface itself may cause an insulative layer to be formed at the interface between the metal particles and the metal surface immediately or after latency.
In the case of tin coated copper particles in the adhesive the material must be heated hot enough to reflow the tin to form the internal joints in the polymer which makes this particular type of conductive adhesive undesirable for many applications. Also, in the case of silver particles, attempts at rebonding or rework of the bond in case such is required often results in poor electrical interconnections, necessitating the complete removal of the adhesive and forming a new bond.
A method of providing an isotropic electrically conductive structurally strong adhesive bond between two metal surfaces and the resulting structure is provided. The method includes the provision of an adhesive with conductive particles dispersed therein, which particles have exposed palladium thereon. The particles can either be palladium metal itself or can be another metal, preferably silver particles, plated with palladium. The adhesive with the dispersed particles therein is interposed between two metal surfaces which are to be mechanically bonded and electrically connected, and the palladium is grown in situ to form a continuous metal matrix from the particles and the metal surfaces, the palladium forming a dendritic or dendrite structure which penetrates the metal surfaces. Preferably, the in situ growing is done at elevated temperatures between about 200xc2x0 C. to about 240xc2x0 C. and, preferably around 220xc2x0 C., and at a pressure of from about 17 to about 51 Kg/in.sq. and, more preferably, from about 28 to about 40 Kg/in.sq. The resulting structure is characterized in that the palladium from the various particles interdiffuses with itself as well as penetrating and/or diffusing into the metal surfaces to be joined, and when the palladium is coated onto another metal, there is interdiffusion between the other metal of the particle and the palladium so that an essentially unitary structure or metal matrix within the polymer is provided rather than as in conventional prior art wherein there are mechanical interfaces without metallurgical diffusion or significant mechanical penetration.